Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst.

1. Heterogeneous catalysts
Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture.
Fixed bed catalysts is a kind of heterogeneous catalysts.

2. Homogeneous catalysts
Homogeneous catalysts function in the same phase as the reactants.